The field of the disclosure relates generally to electric power converter equipment and, more particularly, to a system and method for operation of multilevel converters.
Many known multilevel power converters are in use throughout various industries and for a variety of purposes for electric power conversion. Specifically, the term “multilevel converter” refers to a converter that can operate in an inverter mode and in a rectifier mode. One technical sector where known multilevel power converters are used is the medium voltage (MV) variable speed drive (VSD) industry, where MV VSDs are commonly deployed in many diverse processing facilities, e.g., industries such as the electric power generation industry. MV VSD's facilitate fast and precise process control with lower energy consumption, both results typically not attainable through devices such as constant speed drive motors.
Another technical sector where known multilevel power converters are used is the electric power transmission and distribution industry. At least some of the known electric power transmission and distribution facilities are physically positioned in a remote geographical region or in an area where physical access is difficult. One example includes electric power transmission and distribution facilities geographically located in rugged and/or remote terrain, for example, mountainous hillsides, extended distances from electric power grids, and submerged, e.g., off-shore oil and gas exploration and recovery installations. Many of these known electric power transmission and distribution facilities include a separated power conversion assembly, or system, electrically coupled to an alternating current (AC) power source, e.g., a utility power grid. Such known separated power conversion assemblies include a rectifier portion that converts the AC transmitted by the utility power generation grid to direct current (DC) and an inverter portion that converts the DC to AC of a predetermined frequency and voltage amplitude. The rectifier portion and the inverter portion use multilevel power converters that may shift between operating as a rectifier and operating as an inverter. Most known multilevel converters include semiconductor-based switching devices, e.g., thyristors, including insulated gate bipolar transistors (IGBTs). The rectifier and inverter portions are typically electrically coupled via a medium voltage DC (MVDC) or a high voltage DC (HVDC) link.
Various known multilevel converter topologies are in service or have been available for service. Many of the DC links for known multilevel converters include capacitors to facilitate levelizing DC voltage within the DC link to stabilize power transmission between the multilevel converters. These capacitors are referred to as “flying capacitors” herein. The voltages of the flying capacitors vary throughout operation of the associated multilevel converters as the operation of the switching devices in the converter vary. Also, the output voltage pattern and the blocking voltage of each switching device are determined by the flying capacitor voltages. In order to get the appropriate multilevel output with low harmonic distortion and prevent the devices from attaining overvoltage conditions, the flying capacitor voltages are maintained at or near certain voltage levels, which are normally defined as references, or reference voltages for the flying capacitors through all modes of operation of the multilevel converters, including startup. However, the voltage values of the flying capacitors prior to placing the multilevel converters in service is relatively low compared to the operating voltages, and may be as low as zero volts.
As such, the flying capacitor voltages are pre-charged to the references or near references first, before the multilevel converters start to switch. Otherwise, the service life of the switching devices may be shortened by exposure to overvoltage conditions. Traditionally, the pre-charge procedure requires a voltage sensor on each flying capacitor. As the ratings of the voltage levels associated with multilevel converter switching devices increases with the power ratings of the converters, the number of flying capacitors also increases appropriately. Therefore, more voltage sensors are needed which increases the costs of assembling and maintaining the multilevel converters and additional input channel resources associated with the controller further drives up cost.